This invention relates to semiconductor devices generally, and more particularly, to an improved accelerometer or transducer structure capable of converting mechanical movements or displacements into electrical signals.
It is well known in the prior art that a slotted metal cantilever beam will have most of its deflection occurring across the slot. In U.S. Pat. No. 3,351,880 issued to Wilner in Nov. 30, 1976, it is disclosed that if a sensor, whose inherent stiffness is small compared to the stiffness of the remaining material under the slot, is used to span the slot, then the slot displacement will be transferred to the sensor. In certain embodiments, the sensor was cemented or directed across the slot. The resulting strain in the sensor will be the displacement across the slot divided by the slot width. To achieve maximum performance, the slot must be as narrow as possible. However, for small slot widths and in order to obtain sufficient resistance in the sensor, the cross section of the sensor must be very small. Using a sensor of such small cross section makes the device very fragile and difficult to fabricate. To overcome these problems and to be able to affect the overall stiffness of the slotted beam independent of the stiffness of the sensor, Kurtz one of the inventors herein, taught the use of shim members which bridged the slot onto which sensors were affixed using cements or other bonding agents. See U.S. Pat. No. 3,995,247 entitled TRANSDUCERS EMPLOYING GAP BRIDGING SHIM MEMBERS issued to A. D. Kurtz on Nov. 30, 1976, one of the inventors herein. In this way, the overall reliability of the device was improved and the sensor could be designed independently of the slot-spanning shim.
Details of the mechanical performance as regard to sensitivity and frequency response, are provided in U.S. Pat. No. 3,995,247. The major problem with this improvement and earlier devices was the necessity of using slotted metal beams and using bonding agents to affix the sensors either to the beam itself or to the shim. In addition, forming the slot in the metal was difficult, inaccurate and most important, the narrowest slot widths were on the order of 10 to 50 mils (0.001"-0.005").
In U.S. Pat. No. 4,498,229 entitled PIEZORESISTIVE TRANSDUCER issued on Feb. 12, 1985 to Leslie B. Wilner, it is taught that some of these problems can be avoided by forming an integral unsupported sensor spanning the slot obtained by preferential etching. See also U.S. Pat. No. 4,605,919 issued on Aug. 12, 1986 entitled PIEZORESISTIVE TRANSDUCER, to Wilner and which is a divisional of the '229 patent. By utilizing the integral, unsupported sensor disclosed by Wilner, the slot can be made much smaller and there will be no loss in strain transmission because of slip in the cement. However, this approach again makes the sensor the only mechanical spanning member. For the small spanning lengths possible in such designs, the sensor cross sections are exceedingly small, thus again leading to very fragile structures.
It is an object of the present invention to extend the concept of slot-spanning shims into solid state fabrication to achieve a less expensive and more efficient structure.